Internal combustion (IC) engines are typically coupled to an emission control device to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). For lean engine operation of the IC engine, a lean NOx Trap (LNT) can be coupled to the emission control device for reduce exhaust NOx emissions. The LNT stores exhaust components, such as oxygen and NOx, during the lean operation. When the quantity of NOx stored in the LNT exceeds a predetermined threshold value, the LNT undergoes a regeneration process, also referred to as DeNOx regeneration or a purge, the purpose of which is to reduce the nitrogen oxides (NOx) that have accumulated in the LNT. Once the purge is completed, the lean engine operation may resume again. Therefore, the LNT stores exhaust emissions such as, for example, oxidants, during the operation of the engine at a lean air-fuel ratio, and releases and purges the exhaust emissions when the engine is operating at a richer than stoichiometric or stoichiometric air-fuel ratio.
In the current scenario, environmental protection regulations demand that the performance of the LNT is being monitored periodically to prevent excessive NOx emissions. If the LNT deteriorates over time, the ability to trap NOx degrades with a resultant increase in atmospheric emissions. Therefore, it is desirable to monitor the LNT for providing an indication of deterioration or degradation of the LNT beyond a predetermined limit.
An exemplary system for monitoring a LNT is described in US Patent Publication US20160003123 A1 ('123 publication). The '123 publication describes an electronic control module for operating an IC engine. The electronic control module is configured to monitor a first air-fuel equivalence ratio of engine exhaust gases upstream of a NOx trap, and to activate a diagnostic routine for the NOx trap when the first air-fuel equivalence ratio is smaller than one. The diagnostic routine enables the electronic control module to monitor a second air-fuel equivalence ratio of engine exhaust gases downstream of the NOx trap, to use the first and second air-fuel equivalence ratios to calculate an index that is representative of the conversion efficiency of the NOx trap, and to identify a degradation of the NOx trap when the efficiency index is lower than a predetermined threshold value. The '123 publication describes a mechanism which allows NOx trap monitoring using fuel efficient regeneration, in which the fuel efficient regeneration is activated for monitoring one NOx trap. However, the inventors herein have recognized potential disadvantages with the above approach. In one example, in cases where an engine exhaust after-treatment system has more than one NOx trap, then each of the NOx traps need monitoring. As a consequence, a desired rich air-fuel mixture purges may need to be triggered periodically for each NOx trap. The system and method described in the '123 publication does not enable adjusting an air-fuel ratio of exhaust entering a second (or subsequent) NOx trap positioned downstream of a first NOx trap. Therefore, it may not be possible to simultaneously monitor operation of a plurality of NOx traps.
In one example, the issues described above may be addressed by a method comprising: receiving a first exhaust gas of a desired air-fuel ratio upstream of a first lean NOx trap (LNT); initiating a richer than stoichiometric regeneration of the first LNT for obtaining a second exhaust gas downstream of the first LNT; evaluating an air-fuel ratio of the second exhaust gas received downstream of the first LNT; in response to the evaluated air-fuel ratio of the second exhaust gas being higher than the desired air-fuel ratio, activating injection of a vaporized reductant by an injector disposed downstream of the first LNT to the second exhaust gas for obtaining the desired air-fuel ratio of the second exhaust gas, where the injector is a vaporizer; receiving the second exhaust gas of the desired air-fuel ratio upstream of the second LNT; initiating a richer than stoichiometric regeneration of the second LNT for obtaining a third exhaust gas downstream of the second LNT; evaluating an air-fuel ratio of the third exhaust gas received downstream of the second LNT; and determining a working state of each of the first LNT and the second LNT based on each of the desired air-fuel ratio of the first exhaust gas, the evaluated air-fuel ratio of the second exhaust gas, and the evaluated air-fuel ratio of the third exhaust gas. In this way, by adjusting air-fuel ratio of exhaust entering a plurality of LNTs, diagnostics of each LNT may be carried out simultaneously.
The present summary is provided to introduce concepts related to monitoring of an engine exhaust after-treatment system. The concepts are further described below in the detailed description. In one implementation, a method for monitoring an engine exhaust after-treatment system with more than one LNT is described. To this end, the method utilizes a system to perform all steps described below for monitoring the exhaust gas after-treatment system. For monitoring the exhaust gas after-treatment system, a first exhaust gas of a desired first air-fuel ratio is received upstream of a first lean NOx trap (LNT). Once received, a richer than stoichiometric regeneration of the first LNT is initiated for obtaining a second exhaust gas with an air-fuel ratio downstream of the first LNT. The air-fuel ratio of the second exhaust gas received downstream of the first LNT is monitored to check whether the air-fuel ratio of the second exhaust gas is one of lean of stoichiometry, stoichiometric, and rich of stoichiometry. When the air-fuel ratio downstream of the first LNT is higher than the desired air-fuel ratio, a vaporized reductant injection to the second exhaust gas is activated by an injector such as a vaporizer disposed downstream of the first LNT. After the vaporized reductant (such as fuel) injection, a well-controlled (desired) air-fuel ratio of the second exhaust gas downstream of the first LNT is obtained. Further, the air fuel ratio of the second exhaust gas is well-controlled in terms of stability and their threshold value. In one example, the desired air-fuel ratio is at least one of a stoichiometric and an under-stoichiometric ratio.
Further, the second exhaust gas of a desired air-fuel ratio is received upstream of the second LNT, which is coupled downstream of the first LNT. Further, a richer than stoichiometric regeneration of the second LNT is initiated. Once the second LNT is regenerated, a third exhaust gas of a third air-fuel ratio received downstream of the second LNT is evaluated. Finally, based on the evaluation of the air-fuel ratios upstream and downstream of that LNT, a working state of a respective LNT is determined. In one example, the air-fuel ratio monitored downstream of respective LNT may be compared with a predefined threshold value. Based on the comparison, the working state of the LNT can be determined.
Thus, by applying the previously described method, more than one LNT can be monitored simultaneously using a single purge and therefore minimizing the fuel consumption. Additionally, the method utilizes the single purge for monitoring of the subsequent LNTs by injecting the vaporized reductant to the exhaust gas upstream of the subsequent LNT in order to bring the air fuel ratio of the exhaust gas to a desired air-fuel ratio to perform subsequent purging, therefore ensuring efficient monitoring of the plurality of LNTs using minimized quantity of fuel. By vaporizing the reductant, a smaller quantity of fuel may be used to achieve the desired air-fuel ratio upstream of the second LNT within a shorter duration, thereby facilitating simultaneous regeneration and diagnostics of the two consecutive LNTs.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.